Multi-Layer PZT Microactuator Having A Poled But Inactive PZT Constraining Layer

ABSTRACT

A multi-layer piezoelectric microactuator assembly has at least one poled and active piezoelectric layer and one poled but inactive piezoelectric layer. The poled but inactive layer acts as a constraining layer in resisting expansion or contract of the first piezoelectric layer thereby reducing or eliminating bending of the assembly as installed in an environment, thereby increasing the effective stroke length of the assembly. Poling only a single layer would induce stresses into the device; hence, polling both piezoelectric layers even though only one layer will be active in use reduces stresses in the device and therefore increases reliability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/055,618 filed Feb. 28, 2016, which is a continuation of U.S.patent application Ser. No. 14/672,122 filed Mar. 28, 2015, now U.S.Pat. No. 9,330,698, which is a continuation-in-part of U.S. patentapplication Ser. No. 14/214,525 filed Mar. 14, 2014, now U.S. Pat. No.9,117,468, which claims the benefit of U.S. Provisional PatentApplication No. 61/802,972 filed Mar. 18, 2013 and of U.S. ProvisionalPatent Application No. 61/877,957 filed Sep. 14, 2013. application Ser.No. 14/672,122 is also a continuation-in-part of U.S. patent applicationSer. No. 14/566,666 filed Dec. 10, 2014, now U.S. Pat. No. 9,330,694,which claims benefit of U.S. Provisional Patent Application No.62/061,074 filed Oct. 17, 2014. application Ser. No. 14/672,122 alsoclaims the benefit of U.S. Provisional Patent Application No. 62/085,471filed Nov. 28, 2014. All of those applications are incorporated byreference as if set forth fully herein.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to the field of suspensions for hard disk drives.More particularly, this invention relates to the field of multi-layerpiezoelectric microactuator having one or more active piezoelectricconstraining layers for use in a dual stage actuated suspension.

2. Description of Related Art

Magnetic hard disk drives and other types of spinning media drives suchas optical disk drives are well known. FIG. 1 is an oblique view of anexemplary prior art hard disk drive and suspension for which the presentinvention is applicable. The prior art disk drive unit 100 includes aspinning magnetic disk 101 containing a pattern of magnetic ones andzeroes on it that constitutes the data stored on the disk drive. Themagnetic disk is driven by a drive motor (not shown). Disk drive unit100 further includes a disk drive suspension 105 to which a magnetichead slider (not shown) is mounted proximate the distal end of load beam107. The “proximal” end of a suspension or load beam is the end that issupported, i.e., the end nearest to base plate 12 which is swaged orotherwise mounted to an actuator arm. The “distal” end of a suspensionor load beam is the end that is opposite the proximal end, i.e., the“distal” end is the cantilevered end.

Suspension 105 is coupled to an actuator arm 103, which in turn iscoupled to a voice coil motor 112 that moves the suspension 105arcuately in order to position the head slider over the correct datatrack on data disk 101. The head slider is carried on a gimbal whichallows the slider to pitch and roll so that it follows the proper datatrack on the spinning disk, allowing for such variations as vibrationsof the disk, inertial events such as bumping, and irregularities in thedisk's surface.

Both single stage actuated disk drive suspensions and dual stageactuated (DSA) suspension are known. In a single stage actuatedsuspension, only the voice coil motor 112 moves suspension 105.

In a DSA suspension, as for example in U.S. Pat. No. 7,459,835 issued toMei et al. as well as many others, in addition to a voice coil motor 112which moves the entire suspension, at least one additional microactuatoris located on the suspension in order to effect fine movements of themagnetic head slider and to keep it properly aligned over the data trackon the spinning disk. The microactuator(s) provide finer control andmuch higher bandwidth of the servo control loop than does the voice coilmotor alone, which only effects relatively coarse movements of thesuspension and hence the magnetic head slider. A piezoelectric element,sometimes referred to simply as a PZT, is often used as themicroactuator motor, although other types of microactuator motors arepossible.

FIG. 2 is a top plan view of the prior art suspension 105 in FIG. 1. TwoPZT microactuators 14 are affixed to suspension 105 on microactuatormounting shelves 18 that are formed within base plate 12, such that thePZTs span respective gaps in base plate 12. Microactuators 14 areaffixed to mounting shelves 18 by epoxy 16 at each end of themicroactuators. The positive and negative electrical connections can bemade from the PZTs to the suspension's flexible wiring trace and/or tothe plate by a variety of techniques. When microactuator 14 isactivated, it expands or contracts and thus changes the length of thegap between the mounting shelves thereby producing fine movements of theread/write head that is mounted at the distal end of suspension 105.

FIG. 3 is a side sectional view of the prior art PZT microactuator andmounting of FIG. 2. Microactuator 14 includes the PZT element 20 itself,and top and bottom metalized layers 26, 28 on the PZT which defineelectrodes for actuating the PZT. PZT 14 is mounted across a gap byepoxy or solder 16 on both its left and right sides as shown in thefigure.

In DSA suspensions it is generally desirable to achieve a high strokedistance from the PZT per unit of input voltage, or simply “strokelength” for short.

Many DSA suspension designs in the past have mounted the PZTs at themount plate. In such a design, the linear movement of the PZTs ismagnified by the length of the arm between the rotation center of thePZTs and the read/write transducer head. A small linear movement of thePZTs therefore results in a relatively large radial movement of theread/write head.

Other suspension designs mount the PZT at or near the gimbal. An exampleof a gimbal-mounted PZT is the DSA suspension shown in U.S. Pat. No.8,879,210 which is assigned to the assignee of the present invention. Ina gimbal-mounted DSA suspension (a “GSA” suspension) it is particularlyimportant to achieve high stroke length, because those designs do nothave nearly as long an arm length between the PZTs and the read/writetransducer head. With a shorter arm length, the resulting movement ofthe read/write head is correspondingly less. Thus, achieving a largestroke length is particularly important in GSA design.

SUMMARY OF THE INVENTION

The inventors of the application have identified a source of lost PZTstroke length in a suspension having a PZT microactuator mounted theretoaccording to the prior art, and have developed a PZT microactuatorstructure and method of producing it that eliminates the source of thatlost stroke length.

FIG. 4A is a side sectional view of a PZT microactuator 14 mounted in asuspension according to prior art FIG. 2, when the PZT is actuated by adriving voltage applied thereto in order to expanded the PZT. Becausethe bottom layer 22 of the PZT is partially constrained by being bondedto the suspension 18 on which it is mounted, the bottom layer 22 doesnot expand in a linear direction as much as does the top layer 24.Because the top layer 24 expands more than does the bottom layer 22, thePZT 14 bends downward and assumes a slightly convex shape when viewedfrom the top. The resulting loss in linear stroke length is shown in thefigure as δ1.

FIG. 4B shows the PZT microactuator 14 of FIG. 4A when the PZT isactuated by a driving voltage applied thereto to contract the PZT.Because the bottom layer 22 of the PZT is partially constrained by beingbonded to the suspension 18 on which it is mounted, the bottom layer 22does not contract in a linear direction as much as does the top layer24. Because the top layer 24 contracts more than does the bottom layer22, the PZT 14 bends upward and assumes a slightly concave shape whenviewed from the top. The resulting loss in linear stroke length is shownin the figure as δ2.

Thus, although purely linear expansion and contraction of the PZT uponactuation is desired, in the conventional mounting the PZT experiencesbending either up or down which results in lost stroke length.

FIG. 5 is a diagram and an associated equation for the amount ofeffective linear stroke added or lost due to bending of a PZT. When thebeam bends up as shown in FIG. 4A the bottom tip point will have apositive displacement δ in the x-direction when the bending angle issmall.

FIG. 6 is a plot of stroke loss due to bending verses bending angle forthree different thicknesses of PZTs. As shown in the figure, for a PZTwith a length of 1.50 mm and a thickness of 45 μm, the bending causes apositive x-displacement δ when the bending angle is less than 5 degrees.For that amount of bending, it can also been seen that thicker beamsproduce greater x-displacement than do thinner beams. Similarly, whenthe PZT contracts under the applied voltage, the right half of the PZTbends downward, and the bottom tip of the PZT which is bonded to thesuspension will experience a negative x-displacement. In other words, inthe conventional mounting of a PZT onto a suspension, the component δ oflinear displacement due to bending is in the opposite direction as theactuation of the PZT. It would therefore be desirable to reduce oreliminate that delta, or to even reverse the sign of that delta so thatthe net result is that the amount of total linear expansion orcontraction is actually increased.

The present invention is of a PZT element that has one or more stiffrestraining layers or restraining elements bonded onto at least one sideor face opposite the side or face on which the PZT is mounted to thesuspension, in order to reduce, eliminate, change the direction of, orotherwise control bending of the PZT when it is actuated. Thecounterintuitive result is that even though the PZT has a stiff layeradded to it that, at least nominally, restrains the expansion andcontraction of the PZT, the effective linear stroke distance achievedactually increases. A PZT having a restraining layer according to theinvention can be used as a microactuator in a hard disk drivesuspension, although it could be used in other applications as well.

In a preferred embodiment the effect of the restraining layer is toactually change the direction of bending. Thus, for a PZT that is bondedon its bottom surface to the suspension, the presence of the restraininglayer has the effect that when the piezoelectric element is actuated bya voltage that causes the piezoelectric element to expand, thepiezoelectric element bends in a direction that causes the top face tobecome net concave in shape; and when the piezoelectric element isactuated by a voltage that causes the piezoelectric element to contract,the piezoelectric element bends in a direction that causes the top faceto become net convex in shape. The effect is therefore to actuallyincrease the effective linear expansion in expansion mode, and toincrease the effective linear contraction in contraction mode. Thepresence of the restraining layer therefore actually increases theeffective stroke length.

The PZT with its constraining layer can be manufactured by varioustechniques including laminating the constraining layer to an existingPZT element, or one of the PZT element and the constraining layer can beformed on top of the other by an additive process. Such an additiveprocess can include depositing a thin film PZT onto a substrate such asstainless steel (SST). The constraining layer can be stainless steel,silicon, ceramic such as substantially unpoled (unactivated) ceramicmaterial of otherwise the same ceramic material as constitutes the PZTelement, or another relatively stiff material. If the restraining layeris non-conductive, one or more electrical vias comprising columns ofconductive material can be formed through the restraining layer in orderto carry the activating voltage or ground potential from the surface ofthe microactuator to the PZT element inside.

The constraining layer may be larger (of greater surface area) than thePZT element, the same size as the PZT element, or may be smaller (oflesser surface area) than the PZT element. In a preferred embodiment,the constraining layer is smaller than the PZT element, giving themicroactuator a step-like structure with the shelf of the step uncoveredby the restraining layer, and with the shelf being where the electricalconnection is made to the PZT element. One benefit of such aconstruction including a shelf where the electrical connection is madeis that the completed assembly including the electrical connection has alower profile than if the restraining layer covers the entire PZT. Alower profile is advantageous because it means that more hard driveplatters and their suspensions can be stacked together within a givenplatter stack height, thus increasing the data storage capacity within agiven volume of disk drive assembly.

Simulations have shown that microactuators constructed according to theinvention exhibit enhanced stroke sensitivity, and also exhibitedreduced sway mode gain and torsion mode gain. These are advantageous inincreasing head positioning control loop bandwidth, which translatesinto both lower data seek times and lower susceptibility to vibrations.

An additional advantage of adding a constraining layer(s) or element(s)to the PZT according to the invention is that in hard disk drives today,the suspension and its components including the PZT are usually verythin. Microactuators used in today's DSA suspension designs in which thePZTs are mounted at the mount plate, are on the order of 150 μm thick.In gimbal-mounted DSA suspension designs the PZTs are even thinner,often being less than 100 μm thick. The PZT material is therefore verythin and brittle, and can crack easily during bothmanufacturing/assembly, including both the process of manufacturing thePZT microactuator motor itself as well as the automated pick-and-placeoperation in the suspension assembly process. It is expected that PZTsin future generation hard drives will be 75 μm thick or thinner, whichwill exacerbate the problem. It is anticipated that PZTs this thin willnot only be susceptible to damage during manufacturing/assembly, butcould also be vulnerable to cracking or breaking when the disk driveexperiences shock, i.e., g-forces. The additional stiff, resilientconstraining layer according to the present invention providesadditional strength and resiliency to the PZT thus helping to preventthe PZT from cracking or otherwise mechanically failing duringmanufacturing/assembly and during shock events.

In another aspect of the invention, a microactuator assembly is amulti-layer PZT device, with multiple active PZT layers including one ormore active PZT layers acting as restraining layers that tend tocounteract the action of the main active PZT layer.

The idea that by adding one or more layers that resist the main PZTlayer's movement, overall net stroke length can be increased, iscounterintuitive. It is even more counterintuitive to think that byadding one or more active layers that actively act in the oppositedirection as the main PZT layer, overall net stroke length can beincreased even further. Nevertheless, that is the result which thepresent inventors have demonstrated.

In a further embodiment, the invention is of a multi-layer PZTmicroactuator assembly in which the constraining layer comprises poledbut inactive piezoelectric material. The inventors have found thatbecause poling piezoelectric materials changes its dimensions, polingonly one layer of a multi-layer device introduces strains into thedevice which could render the device more prone to cracking and hencefailure. The reliability of the device is therefore increased by polingnot only the main or active PZT layer that is closest to the surface ofthe suspension assembly or other environment to which the assembly isbonded, but also the inactive layer that faces away from the environmentto which the assembly is bonded.

Exemplary embodiments of the invention will be further described belowwith reference to the drawings, in which like numbers refer to likeparts. The drawing figures might not be to scale, and certain componentsmay be shown in generalized or schematic form and identified bycommercial designations in the interest of clarity and conciseness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective view of a prior art magnetic hard diskdrive.

FIG. 2 is a top plan view of the suspension of the disk drive of FIG. 1.

FIG. 3 is a side sectional view of the prior art PZT microactuator andmounting of FIG. 2.

FIG. 4A is a side sectional view of a PZT microactuator mounted in asuspension according to prior art FIG. 2 when a voltage is applied tothe PZT so as to expand it.

FIG. 4B is a side sectional view of a PZT microactuator mounted in asuspension according to prior art FIG. 2 when a voltage is applied tothe PZT so as to contract it.

FIG. 5 is a diagram and an associated equation for the amount of linearstroke added or lost due to bending of a PZT.

FIG. 6 is a plot of stroke loss due to bending verses bending angle forthree different thicknesses of PZTs.

FIG. 7 is a side sectional view of a PZT having a constraining layerbonded thereto in accordance with the present invention.

FIG. 8A is a side sectional view of the PZT microactuator of FIG. 8 whena voltage is applied to the PZT so as to expand it.

FIG. 8B is a side sectional view of a PZT microactuator of FIG. 8 when avoltage is applied to the PZT so as to contract it.

FIG. 9 is a graph showing stroke length per unit input voltage in unitsof nm/V verses constraining layer thickness, for a PZT that is 130 μmthick.

FIG. 10 is a side elevation view of a PZT having a constraining layerbonded thereto according to the invention.

FIG. 11 is a graph of stroke length vs. PZT thickness for the PZT ofFIG. 10 where the combined thickness of the PZT and the restraininglayer is kept constant at 130 μm.

FIG. 12 is a graph of GDA stroke sensitivity versus constraining layerthickness for a suspension having a PZT with stainless steelconstraining layers of varying thicknesses.

FIGS. 13(a)-13(h) illustrate one manufacturing process by which a PZThaving a constraining layer according to the invention can be produced.

FIGS. 14(a) and 14(b) are oblique views of a GSA suspension beingassembled with thin film PZT microactuator motors according to theinvention.

FIG. 15 is cross-sectional view of the microactuator area of FIG. 14(b)taken along section line B-B′.

FIG. 16 is a graph of stroke sensitivity versus SST substrate thicknessfor the microactuator of FIG. 15 according to a simulation.

FIGS. 17(a)-17(f) illustrates a process for manufacturing a thin filmPZT structure having a stainless steel substrate according to theinvention.

FIG. 18 is a top plan view of a thin film PZT structure having a siliconsubstrate according to the invention.

FIG. 19 is a side sectional view of the thin film PZT structure of FIG.18 taken along section line A-A′.

FIG. 20 is a graph of stroke sensitivity versus silicon substratethickness for the microactuator of FIG. 19 according to a simulation.

FIGS. 21(a)-21(e) illustrate a process for manufacturing the thin filmPZT structure of FIG. 18.

FIG. 22 is a top plan view of a thin film PZT having a substrate andhaving side vias according to an embodiment of the invention.

FIG. 23 is a sectional view of the microactuator of FIG. 22 taken alongsection line A-A′.

FIG. 24 is a sectional view of a PZT microactuator according to anadditional embodiment of the invention.

FIG. 25 is an oblique view of a GSA suspension having a pair of the PZTmicroactuators of FIG. 24.

FIG. 26 is a sectional view of the GSA suspension of FIG. 25 taken alongsection line A-A′.

FIG. 27 is a graph of the PZT frequency response function of thesuspension of FIG. 25 according to a simulation.

FIGS. 28(a)-28(j) illustrate an exemplary process for manufacturing thePZT microactuator assembly of FIG. 24.

FIG. 29 is a side sectional view of a multi-layer PZT microactuatorassembly according to an additional embodiment of the invention in whichthe PZT is a multi-layer PZT.

FIG. 30 is a side sectional view of a multi-layer PZT microactuatorassembly according to an additional embodiment of the invention in whichan extra thick electrode acts as the restraint layer.

FIG. 31 is a cross sectional view of an embodiment in which therestraining layer of the microactuator assembly comprises one or moreactive PZT layers tending to act in the opposite direction as the mainactive PZT layer.

FIG. 32 shows the poling of the microactuator assembly of FIG. 31,including the resulting poling directions of the various layers ofactive PZT material.

FIG. 33 is an exploded view of the microactuator assembly of FIG. 31,showing conceptually the electrical connections.

FIG. 34 is a graph showing the stroke sensitivity (in nm/V) ofmicroactuators having one or more active restraining layers according tosimulations, for various constructions.

FIG. 35 is cross sectional view of another embodiment in which themicroactuator assembly comprises multiple active PZT layers, andconceptually showing the poling process and the resulting polingdirections.

FIG. 36 is a side sectional view of a multi-layer PZT microactuatorassembly according to a further embodiment, in which the PZT is amulti-layer PZT that is commonly poled and in which the top layer isthicker than the bottom layer.

FIG. 37 is a side sectional view of the PZT microactuator assembly ofFIG. 36 illustrating the assembly installed within a suspension.

FIG. 38 is a side sectional view of a multi-layer PZT microactuatorassembly according to a still further embodiment, in which the PZT is amulti-layer PZT having three separate poles, and the PZT is reversepoled.

FIG. 39 is a side sectional view of the PZT microactuator assembly ofFIG. 38 illustrating the assembly installed within a suspension.

FIG. 40 is a graph showing the stroke sensitivity of the PZTmicroactuator assemblies of FIGS. 37 and 39 compared to the strokesensitivity of a standard single-layer PZT, as a function of thethickness of the top PZT layer, according to simulations.

FIG. 41 is a side sectional view of the PZT microactuator assembly ofFIG. 36 illustrating the assembly installed within a suspensionaccording to an alternative assembly method.

FIG. 42 is a side sectional view of a PZT microactuator according to afurther embodiment having a poled but inactive constraining layer.

FIG. 43 is a side sectional view of the PZT microactuator assembly ofFIG. 42 illustrating the assembly installed within a suspension.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 7 is a side sectional view of a PZT microactuator assembly 114having a constraining layer 130 bonded thereto in accordance with anembodiment of present invention. In keeping with the orientation shownin the figure, the side of the PZT which is bonded to the suspensionwill be referred to as the bottom side 129 of PZT 114, and the side ofthe PZT away from the side at which the PZT is bonded to the suspensionwill be referred to as the top side 127. According to the invention, oneor more constraining layers or constraining elements 130 is bonded tothe top side 127 of microactuator PZT element 120. The constraininglayer 130 preferably comprises a stiff and resilient material such asstainless steel and is preferably bonded directly to the top surface 127of the PZT element 120 including its top electrode 126, or the SSTmaterial may itself serve as the top electrode thus making itunnecessary to separately metalize the top surface. The constraininglayer 130 is stiff enough so as to significantly reduce, eliminate, oreven reverse the bending of the PZT when actuated. The SST layer 130preferably has a layer 131 of gold or other contact metal in order toensure a good electrical connection to the SST.

Alternatively, instead of the constraining layer 130 being stainlesssteel, it could be ceramic such as an unactivated (unpoled, orunpolarized) layer of the same ceramic material as forms thepiezoelectric layer 120, and could be integrated into the assembly byeither bonding or by deposition. The ceramic material is unpolarizedmeaning that it exhibits substantially less piezoelectric behavior, suchas less than 10% as much piezoelectric behavior, as the poled ceramicthat defines piezoelectric layer 120. Such an assembly, defining a stackconsisting from the bottom up of electrode/poled PZT/electrode/unpoledPZT, may be easier to manufacture than a stack ofelectrode/PZT/electrode/SST.

In the discussion that follows, for simplicity of discussion top andbottom electrodes 126, 128 are sometimes omitted from the figures andfrom the discussion, it being understood that PZT microactuators willalmost always have at least some type of top and bottom electrode.

A layer of copper or nickel may be deposited onto the SST layer 130before gold layer 131 is applied in order to increase the adhesion ofthe gold to the SST, as discussed in U.S. Pat. No. 8,395,866 issued toSchreiber et al. which is owned by the assignee of the presentapplication, and which is hereby incorporated by reference for itsteaching of electrodepositing other metals onto stainless steel.Similarly, the electrodes 126,128 may comprise a combination of nickeland/or chromium, and gold (NiCr/Au).

124-167 (FIG. 5). In one illustrative embodiment according to asimulation, the thicknesses of the various layers were:

130 PZT 3 μm 126, 128, 131 NiCr/Au 0.5 μm

The thin film PZT had a length of 1.20 mm, the PZT bonding had a widthof 0.15 mm at both ends, and the piezoelectric coefficient d31 was 250pm/V. In some embodiments, the SST layer may be at least 12 micrometersthick in order to provide adequate support.

In the above example the DSA suspension exhibited a stroke sensitivityof 26.1 nm/V according to a simulation. In contrast, a 45 μm thick bulkPZT (d31=320 pm/V) with the same geometry would typically exhibit astroke sensitivity of only 7.2 nm/V.

The ratio of thicknesses of the SST layer to the PZT layer may be ashigh as 1:1, or even 1.25:1 or even higher. As the thickness ratio ofthe constraint layer to the PZT reaches approximately 1:25, the strokesensitivity improvement due to the constraint layer may start to benegative, indicating the thickness limitation of the PZT constraintlayer.

FIG. 8A is a side sectional view of a PZT microactuator 114 of FIG. 7when a voltage is applied to the PZT so as to expand it. The PZT strokeconsists of two vectors, one that is the pure extension stroke δe, theother is the extension contribution M due to the constraining layercausing the PZT's right tip to bend upward, instead of bending downwardas would be the case without the restraining layer. The total strokelength is δe+δ1. In expansion mode therefore, the PZT assumes a slightlyconcave shape when viewed from the top, i.e. the PZT top surface assumesa slightly concave shape, which is in a bending direction that is theopposite from the bending of the prior art PZT of FIG. 4. That bendingaccording to the invention therefore adds to the effective stroke lengthrather than subtracting from it.

FIG. 8B is a side sectional view of a PZT microactuator of FIG. 7 when avoltage is applied to the PZT 114 so as to contract it. The PZT strokeconsists of two vectors, one that is the pure contraction stroke −|δc|,the other is the contraction contribution δ2 due to the constraininglayer causing the PZT's right tip to bend downward, instead of bendingupward as would be the case without the restraining layer. The totalstroke length is −[δc+δ2]. In contraction mode therefore, the PZTassumes a slightly convex shape when viewed from the top, i.e. the PZTtop surface assumes a slightly convex shape, which is in a bendingdirection that is the opposite from the bending of the prior art PZT ofFIG. 4. That bending according to the invention therefore adds to theeffective stroke length rather than subtracting from it.

Adding the constraining layer 130 to a PZT microactuator 114 has noappreciate affect on the stroke length for the otherwise unrestrainedand unbonded PZT 114. When that PZT 114 is bonded to a suspension 18 atits bottom ends such as shown in FIG. 4, however, the effect of theconstraining layer is actually to slightly increase the stroke length.Stainless steel has a Young's modulus of around 190-210 GPa. Preferablythe material for the constraining layer has a Young's modulus of greaterthan 50 GPa, and more preferably greater than 100 GPa, and morepreferably still greater than 150 GPa.

FIG. 9 is a graph of stroke length per unit input voltage in units ofnm/V verses constraining layer thickness, for a PZT 114 that is 130 μmthick and has a constraining layer 130 of stainless steel bondedthereto, according to a simulation. Adding an SST restraining layer of20 μm, 40 μm, and 60 μm thick onto the PZT top surface each result in anincreased total stroke length. Adding a constraining layer thereforeactually increased the total stroke length.

One could also hold constant the total combined thickness of the PZT andthe constraining layer, and determine an optimal thickness for theconstraining layer. FIG. 10 is a side elevation view of a combined PZTand constraining layer bonded thereto according to the invention, inwhich the total thickness is kept constant at 130 μm. FIG. 11 is a graphof stroke length vs. PZT thickness for the PZT of FIG. 10 where thecombined thickness of the PZT and the restraining layer is kept constantat 130 μm according to a simulation. With no constraining layer, the 130μm thick PZT has a stroke length of approximately 14.5 nm/V. With aconstraining layer 130 thickness of 65 μm and a PZT thickness of 65 μm,the PZT has a stroke length of approximately 20 nm/V. Adding theconstraining layer therefore actually increased the effective strokelength by approximately 35%.

FIG. 12 is a graph of GDA stroke sensitivity versus constraining layerthickness for a GDA suspension having the microactuator of FIG. 7 for aPZT element that is 45 μm thick and a stainless steel constraining layeron top of varying thicknesses, according to a simulation. As seen in thegraph, a constraining layer that is 30 μm thick increased the GDA strokesensitivity from 9 nm/V to slightly more than 14.5 μm, which representsan increase in stroke length of greater than 50%.

FIGS. 13(a)-13(h) illustrate one manufacturing process by which a PZTmicroactuator assembly having a constraining layer according to theinvention can be produced. This method is an example of an additivemethod in which the PZT material is deposited onto a substrate that willbe the constraint layer. The process begins with a first substrate 140as shown in FIG. 13(a). In FIG. 13(b) a first UV/thermal tape 142 isapplied to the substrate. In FIG. 13(c) a pre-formed SST layer 130 isadded onto the tape. In FIG. 13(d) an electrode layer 126 is depositedonto the SST such as by sputtering or other well known depositionprocesses. In FIG. 13(e) a PZT layer 120 is formed on the electrodelayer by the sol-gel method or other known methods. In FIG. 13(f) asecond electrode 128 is deposited onto the exposed side of the PZT suchas by sputtering. In FIG. 13(g) the SST layer 130 is separated from thetape, and the product is flipped over onto a second tape 143 and asecond substrate 141. In FIG. 13(h) the product is then diced such as bymechanical sawing or laser cutting in order to singulate the individualmicroactuators 114. This process produces a microactuator 114 in whichthe PZT element 120 including its electrodes is bonded directly to theSST restraining layer 130 without any other material, such as an organicmaterial such as polyimide that would render the restraining effect ofthe restraining layer less effective, therebetween. The electrode layersmay be of materials such as Au, Ni, Cr, and/or Cu. Au has a Young'smodulus of about 79 GPA, Cu has a Young's modulus of about 117 GPa, Nihas a Young's modulus of about 200, and Cr has a Young's modulus ofabout 278. Preferably, there is no intermediary layer between the SSTrestraining layer 130 and the PZT element 120 that has a Young's modulusthat is less than 20 GPa, or which has a Young's modulus that issubstantially less than the Young's modulus of the restraining layer, ora Young's modulus that is less than half the Young's modulus of therestraining layer.

Although other methods could be used to produce the product such as bybonding the restraining layer directly to the PZT surface by adhesivesuch as epoxy, the method shown in FIGS. 13(a)-13(g) is currentlyanticipated to be the preferred method.

The SST restraining layer 130 acts as a substrate for the PZT layer 120both during the additive manufacturing process as well as in thefinished product. The restraining layer 130 is therefore sometimesreferred to as a substrate.

FIGS. 14(a) and 14(b) are oblique views of a gimbal mounted dual stageactuated (GSA) suspension 150 being assembled with thin film PZTmicroactuator motors 114 according to the invention. In a GSA suspensionthe PZTs are mounted on the trace gimbal which includes a gimbalassembly, and act directly on the gimbaled area of the suspension thatholds the read/write head slider 164. FIG. 14(a) shows the suspension150 before PZT microactuator assemblies 114 are attached. Each of twomicroactuators 114 will be bonded to, and will span the gap 170 between,tongue 154 to which the distal end of microactuator 114 will be bonded,and portion 156 of the trace gimbal to which a proximal end ofmicroactuator 114 will be bonded. FIG. 14(b) shows the suspension 150after PZT microactuators 114 are attached. When microactuator assembly114 is activated, it expands or contracts and thus changes the length ofthe gap 170 between the tongue 154 and portion 156 of the trace gimbal,thus effecting fine positioning movements of head slider 164 whichcarries the read/write transducer.

FIG. 15 is cross-sectional view of FIG. 14(b) taken along section lineB-B′. GSA suspension 150 includes a trace gimbal 152, which includeslayers of stainless steel, an insulator 157 such as polyimide, and alayer of signal conducting traces 158 such as Cu covered by a protectivemetal 159 such as Au or a combination of Ni/Au. Microactuators 114 areattached at their distal ends to stainless steel tongues 154 extendingfrom the gimbal area by conductive adhesive 162 such as epoxy containingAg particles to make it electrically conductive, and at their proximalends to a mounting area 156 of the stainless steel by non-conductiveadhesive 161 such as non-conductive epoxy. The driving voltageelectrical connection is made by a spot of conductive adhesive 160 thatextends from the gold plated copper contact pad 158 to the top surfaceof PZT microactuator 114, and more particularly in this case to the SSTlayer 130 which constitutes the top electrode of the microactuator.

The SST substrate thickness may be varied to some degree withoutcompromising the benefits of the disclosed thin film PZT structure. FIG.16 is a graph of stroke sensitivity versus SST constraining layerthickness for the microactuator of FIG. 15 according to a simulation. Athin film PZT having a 40 μm thick SST constraining layer exhibited astroke sensitivity of 20 nm/V according to a simulation, which is almost3 times the stroke sensitivity of the aforementioned 45 μm thick bulkPZT. A 45 μm thick SST constraining layer, however, would provide betterprotection to the thin film PZT microactuator.

FIGS. 17(a)-17(f) illustrates an alternative process for manufacturing athin film PZT structure having an SST constraining layer according tothe invention. In FIG. 17(a) the process begins with a silicon substrate144 instead of a substrate 140 and tape 142 as in FIG. 18(b). In FIG.17(b) an SST layer (130) is bonded to the silicon. The process otherwiseproceeds in essentially the same way as the process of FIGS.13(c)-13(h), including the flipping of the assembly over and removal ofthe silicon substrate in FIG. 17(e). Additionally, these figuresexplicitly show the addition of final NiCr/Au layer 131, which was notexplicitly shown in FIG. 13(e).

As mentioned above, different types of constraint layers may be used indifferent implementations. Other rigid materials, either conductive ornon-conductive, can also be used as the constraint layer or substrate.Silicon, for example, could be used as the constraint layer material.FIG. 18 is a top plan view of a thin film PZT structure having a siliconconstraint layer according to an embodiment of the invention. FIG. 19 across sectional view of the microactuator of FIG. 18 taken along sectionline A-A′. Because the silicon constraint layer 230 is non-conductive, avia 232 is provided in order to conduct the PZT driving voltage from aconductive top layer 234 such as Au over silicon 230 through to themetalized electrode 126 on the PZT element 120. The via may be formedand filled with conductive metal as disclosed in U.S. Pat. No. 7,781,679issued to Schreiber et al. which is owned by the assignee of the presentinvention and which is incorporated herein by reference for itsteachings of conductive vias and methods of forming conductive vias.

FIG. 20 is a graph of stroke sensitivity versus silicon substratethickness for the microactuator of FIG. 19 according to a simulation. Asshown in the graph, a thin film PZT having a thickness of 3 μm and a 20μm thick silicon substrate may exhibit a stroke sensitivity of 31.5nm/V. This is more than 4 times as high as the stroke sensitivity of adesign with a 45 μm thick bulk PZT. The silicon substrate also helps toimprove the reliability of the thin film PZT.

FIGS. 21(a)-21(e) illustrate a process for manufacturing the thin filmPZT structure of FIG. 18. The process begins in FIGS. 21(a) and 21(b)with a silicon substrate having a hole or via 232 that has been formedin it such as by laser drilling. In FIG. 21(c) the NiCr/Au layer isadded to silicon substrate 230 to form top electrode 126. The NiCr/Aualso fills the hole to make it an electrical via 232. More generally,other conductive material may be used to fill the via. In FIG. 22(d) aPZT thin film 120 is deposited such as by the sol-gel method, andanother layer of NiCr/Au is added to form the bottom electrode 128. InFIG. 22(e) the material is flipped over, and a final NiCr/Au layer 131is added. Layers 131 and 126 are electrically connected by via 232 sothat a voltage (or ground potential) applied to the conductive goldlayer 131 will be transmitted to the PZT element 126. This manufacturingprocess for a thin film PZT microactuator having a silicon substrate maybe less complicated than the manufacturing process for the thin film PZThaving an SST substrate.

In an alternative embodiment, the middle via on the silicon substratecan be replaced by one or more vias at the end the silicon. Therefore,after the final dicing, a half-circle will be formed at each end of thesilicon. FIG. 22 is a top plan view of a thin film PZT microactuatorhaving a silicon or other non-conductive constraining layer 330 withconductive top layer 231 such as a metallization layer thereon, andhaving side vias 234, 236 that electrically connect top layer 231 to topelectrode 126. FIG. 23 is a sectional view of the PZT of FIG. 22 takenalong section line A-A′. The manufacturing process for this embodimentcan be otherwise identical with that of FIGS. 21(a)-21(e).

The constraining layer may be larger (of greater surface area) than thePZT element, the same size as the PZT element, or may be smaller (oflesser surface area) than the PZT element. FIG. 24 is a side sectionalview of a PZT microactuator assembly 414 in which the constraint layer430 is smaller than the PZT element 420, giving the microactuator astep-like structure having a step 434 and an exposed shelf 422 that isuncovered by the restraining layer 430, and with the shelf 422 beingwhere the electrical connection is made to the PZT element 420. Onebenefit of such a construction including a step where the electricalconnection is made is that the completed assembly including theelectrical connection has a lower profile than if the restraining layer430 covers the entire PZT 420. A lower profile is advantageous becauseit means that more hard drive platters and their suspensions can bestacked together within a given platter stack height, thus increasingthe data storage capacity within a given volume of disk drive assembly.It is anticipated that the constraint layer 430 would cover more than50% but less than 95% of the top surface of PZT element 420 in order toaccommodate the electrical connection on shelf 422.

Simulations have shown that microactuators constructed according to theinvention exhibit enhanced stroke sensitivity, and also exhibitedreduced sway mode gain and torsion mode gain. These are advantageous inincreasing head positioning control loop bandwidth, which translatesinto both lower data seek times and lower susceptibility to vibrations.

FIG. 25 is an oblique view of a GSA suspension having a pair of the PZTmicroactuators 414 of FIG. 24.

FIG. 26 is a sectional view of the GSA suspension of FIG. 25 taken alongsection line A-A′. In this embodiment conductive adhesive 460 such asconductive epoxy does not extend over the restraint layer 430. Rather,conductive epoxy 460 extends onto shelf 422 on top of PZT element 420and establishes electrical connection to the PZT 420 and to the overallmicroactuator assembly 414 by that surface. As depicted, the electricalconnection defined by conductive epoxy 460 has an uppermost extent thatis lower than the top surface of the SST restraint layer 430. Moregenerally, regardless of whether the electrical connection is made byconductive adhesive, a wire that is bonded such as by thermosonicbonding, soldering, or other techniques, the electrical connection 461to the microactuator assembly 414 can be made to be no higher than, oreven lower than, the uppermost extent of microactuator 414. This allowsthe microactuator assembly 414 including its electrical connection to beas thin as possible, which in turn allows for a denser stack of datastorage disk platters within the platter stack of a disk drive assembly.

The figure also explicitly shows gold layer 469 over the stainless steelportion 154 of the trace gimbal to which microactuator 414 is mounted.Gold layer 469 provides corrosion resistance and enhanced conductivityto the SST.

In this embodiment as with all of the other embodiments, the restraininglayer and more generally the top surface of the PZT microactuatorassembly, will normally have nothing bonded to it other than anelectrical connection.

FIG. 27 is a graph of the frequency response of the PZT frequencyresponse function of the suspension of FIG. 26, according to asimulation. The suspension exhibited reduced sway mode gain and torsionmode gain as compared to a simulation without the constraint layer 430.These are advantageous in increasing head positioning control loopbandwidth, which translates into both lower data seek times and lowersusceptibility to vibrations.

FIGS. 28(a)-28(j) illustrate a process for manufacturing the thin filmPZT assembly 114 of FIG. 24. In FIG. 28(a) a bulk PZT wafer 420 isplaced onto a transfer tape 422. In FIG. 28(b) the top electrode layer426 is formed such as by sputtering and/or electrodeposition. In FIG.28(c) a mask 436 is placed over parts of top electrode 426. In FIG.28(d) conductive epoxy 432 is applied. In FIG. 28(e) a stainless steellayer that will be constraint layer 430 is applied to the epoxy, whichis then cured. In FIG. 27(f) the mask 436 is removed. In FIG. 27(g) theassembly is flipped over and placed down onto a second transfer tape443. In FIG. 27(h) the bottom electrode layer 428 is formed such as bysputtering and/or electrodeposition. The PZT element 420 is thenpolarized. In FIG. 27(i) the assembly is then flipped over once more toa third transfer tape 444. IN FIG. 28(j) then assembly is singulated bycutting, to produce finished PZT microactuator assemblies 414.

FIG. 29 is a side sectional view of a multi-layer PZT assembly 514according to an additional embodiment of the invention. The assemblyincludes multi-layer PZT element 520, a first electrode 526 that wrapsaround the device, and a second electrode 528, and a constraint layer530 that is bonded to the PZT element 520 by conductive epoxy 532. Thefigure shows a 2-layer PZT device. More generally, the device could bean n-layer PZT device.

FIG. 30 is a side sectional view of a multi-layer PZT microactuatorassembly 614 according to an additional embodiment of the invention inwhich an extra thick electrode acts as the restraint layer. In thisembodiment, PZT element 620 has a top electrode 626 and bottom electrode628. Top electrode 626 includes a thinner first part 622 defining ashelf, and a thicker second part 630 which performs the majority of therestraining function. Step 634 lies at the transition from the thinnerfirst part 622 to the thicker second part 630. Second electrode 626could be applied to PZT element 620 by a deposition process includingmasking to create step 634, or by a deposition process with selectiveremoval of material to create the step. Alternatively, second electrode626 could be a piece of conductive material such as SST that is formedseparately and then bonded to PZT element 620. Top electrode 626 couldtherefore be of the same material, or of a different material, thanbottom electrode 628. Thicker second part 630 could be at least 50%thicker than thinner part 622 and/or second electrode 628, or thickersecond party 630 could be at least twice as thick as thinner part 622and/or second electrode 628. As with the embodiment of FIGS. 24-26, theelectrical connection could be made to the shelf defined by thinner part622, with the electrical connection not extending as high as, or higherthan, the top surface of the thicker part 630 that defines the restraintlayer.

The scope of the invention is not limited to the exact embodimentsshown. Variations will be obvious to those skilled in the art afterreceiving the teachings herein. For example, the restraining layer neednot be stainless steel, but can be some other relatively stiff andresilient material. The restraining layer need not be a single layer ofone material, but could be composed of different layers of differentmaterials. Although the restraining layer can cover the entire surfaceor substantially the entire top surface, the restraining could coverless than the entire surface, e.g., more than 90% of the top surfacearea, more than 75% of the top surface area, more than 50% of the topsurface area, or even more than 25% of the top surface area. Inembodiments having the step feature, the restraint layer is anticipatedto cover less than 95% of the top surface of the microactuator. Theconstraining layer need not be a single integral layer, but couldcomprise multiple pieces such as a plurality of constraining stripsarranged side by side on the top surface of the PZT, with the stripsextending either in the direction of expansion/contraction orperpendicular to it. In one embodiment, the constraining layer couldcomprise two constraining pieces of stainless steel or other materialbonded onto the top surface of the PZT, with the size and location ofthe two constraining pieces and their bonding generally mirroring themounting area of two mounting shelves to which the PZT is bonded on itsbottom surface. When the overall stiffness added by the restraininglayer on the top of the device generally matches the overall stiffnessadded to the bottom of the device by being bonded to the suspension, andthe bonded areas generally mirror each other, the net bending producedshould be zero or close to zero. The result will be a PZT microactuatorthat, as mounted and deployed in a suspension, exhibits virtually nobending upon actuation.

In any and all of the embodiments discussed herein or suggested thereby,the constraining layer could be chosen so as to reduce the PZT bendingthat would otherwise occur during actuation, or it could be chosen so asto eliminate as much as possible any PZT bending, or it could be chosenso as to reverse the sign of the PZT bending. In applications in whichthe PZT(s) will be used as hard disk drive microactuator(s), it isenvisioned that using a constraining layer to reverse the sign of thebending as shown and described in the illustrative examples above willbe desirable in most cases because that increases the effective strokelength. In other applications for PZTs, however, it might not bedesirable to reverse the sign. Thus, the invention can be used ingeneral to control both the direction and the amount of the bending of aPZT, regardless of how the PZT is mounted or otherwise affixed to othercomponents within any particular application. Depending on theapplication and the parameters chosen, the constraining layer can beused to decrease the PZT bending to less than 50% of what it otherwisewould be, or to less than 25% of what it otherwise would be, or toreverse the sign of the bending. When the sign is reversed, a PZT thatis bonded at or near its ends on its bottom surface and which has arestraining layer on top will bend such that its top surface assumes aconcave shape when the PZT is in expansion or extension mode, ratherthan assuming a convex shape as would a similar PZT that does not have arestraining layer. Similarly, the PZT will assume a convex shape whenthe PZT is in contraction mode, rather than assuming a concave shape aswould a similar PZT that does not have a restraining layer.

For various reasons, PZT elements are sometimes pre-stressed in anapplication such that when the PZT is not actuated by any voltage it isalready bent in one direction or another, i.e., it is already eitherconcave or convex. Of course, such pre-stressed PZTs could be used asmicroactuators in the present invention. In such a case, the PZT mightnot bend into a net or absolute concave shape or a net or absoluteconvex shape. For example, if the PZT is pre-stressed so that it alreadyhas a concave shape, upon activation with a positive activation voltagethe device might bend into a more concave shape, and upon activationwith a negative activation voltage the device might bend into a lessconcave shape which might be a nominally flat shape or it might be aconvex shape. Unless specifically delineated therefore, the terms“concave” and “convex” should be understood in relative terms ratherthan in absolute terms.

FIG. 31 is a cross sectional view of an embodiment of a multi-layermicroactuator PZT assembly 3100 in which the restraining layer(s) of themicroactuator assembly comprises one or more active PZT layers 3130,3140 tending to act in the opposite direction as the main active PZTlayer 3120 which is adjacent the surface of the suspension to which themicroactuator 3100 is bonded. The PZT restraining layers 3130, 3140 thusconstrain and actively oppose the action of main PZT layer 3120, andthus can be referred to as “constraining layers” or “opposing layers.”

The PZT layers 3120, 3130, and 3140 are arranged in stacked planarrelationships to one another. The main PZT layer 3120 comprises activePZT area 3121 which was subject to an electric field during poling andhence was poled, and which is subject to an electric field during deviceactivation and hence will expand or contract, and also includes inactivePZT areas 3122 and 3123 which are not subjected to significant electricfields during either poling or activation and hence are notsignificantly piezoelectrically active. The device includes: a first orbottom electrode 3124; a second and top electrode 3126 for the activePZT area; a third electrode 3132 including end 3128 such that electrode3132 both extends between the first active constraining layer 3130 andthe second active constraining layer 3140 and wraps around the end ofthe PZT; and fourth electrode 3142 on top of the second activeconstraining layer 3140 including wrap-around portion 3143 which wrapsaround both the side and the bottom of the device. The device may bebonded to the suspension using conductive adhesive such as conductiveepoxy 3160 mechanically and electrically bonding electrode 3142 to drivevoltage electrical contact pad 158 which provides the microactuatordriving voltage, and using conductive epoxy 3162 which mechanically andelectrically bonds electrodes 3124 and 3128 of the device to groundedpart 154 of the suspension.

To understand the operation of the device, one must understand how thedevice has been poled. FIG. 32 shows the poling of the device of FIG.31, including the resulting poling directions of the various layers ofactive PZT material. Three voltages are applied: a positive voltage(Vp+) is applied to electrode 3124; a negative voltage (Vp−) is appliedto electrode 3128; and ground is applied to electrode 3142. The arrowsin the figure show the resulting poling directions for active PZT layers3120, 3130, and 3140.

Returning to FIG. 31, the figure shows how the device 3100 is connectedin this illustrative embodiment. Conductive epoxy 3162 bridges and thuselectrically gangs electrodes 3124 and 3132, essentially taking what hadbeen a 3-pole device during poling and changing it to a 2-pole device inoperation. The ganging of electrodes could be accomplished by other wellknown means for making electrical connections other than conductiveepoxy 3162, but using the same conductive epoxy 3162 as is used to bondthe device to the suspension assembly accomplishes the ganging functionwithout requiring a separate ganging step.

When a voltage is applied to electrode 3142 that causes main PZT layer3120 to expand in the x-direction (from left to right) as seen in thefigure due to the expansion of active area 3121, the active PZTconstraining layers 3130 and 3140 will contract in the x-direction. Thatis, the two constraining layers 3130, 3140 tend to counteract, or act inthe opposite direction, as the main PZT layer 3120.

Explained in greater detail, when the device is poled as shown in FIG.32, and the device is electrically connected as shown in FIG. 31, thedevice operates as follows. A positive device activation voltage appliedat electrical contact pad 158 and electrode 3142, together withelectrode 3124 being grounded, causes the following reaction. Theactivation voltage applied to main PZT layer 3120 is the opposite of thepolarity during poling. The main PZT layer 3120 therefore contracts inthe z-direction and thus expands in the x-direction. At the same time,the activation voltage is of the same polarity as was applied to the twoconstraining layers 3130, 3140 during poling. Those PZT layers thereforeexpand in the z-direction and thus contact in the x-direction. The twoconstraining layers 3130, 3140 therefore are tending to contract whilethe main PZT layer 3120 is tending to expand in the relevant direction.

The effect of the constraining layers acting in the opposite directionas the main PZT layer is similar to that described earlier with respectto a passive constraining layer such as constraining layer 130 in FIG.10, and similar restraining layers 230, 330, 430, 530, and 630 in otherembodiments discussed above. The action of the active PZT restraininglayers reduces the bending that would otherwise occur due to the mainPZT layer and its mounting (binding) to the suspension, and can evenreverse the sign of the bending, in either case increasing the netdisplacement caused by the microactuator as mounted.

FIG. 33 is an exploded view of the microactuator assembly of FIG. 31,showing conceptually the electrical connections. Optional features thatwere not visible in FIG. 31 and FIG. 32 but are visible in FIG. 33include patterning 3133 on electrode 3132 and a voltage reducer 3144associated with electrode 3142 whose functions are described below.

A thinner microactuator assembly is desired for a number of reasonsincluding: (1) less mass on the suspension, particularly at or near thegimbal in a gimbal-based DSA suspension which is sometimes referred toas a GSA suspension, which in turns means a greater lift-off force asmeasured in g-forces, i.e., a greater resistance to shock; (2) reducedwindage; and (3) greater stack density within the head stack assemblywhich means that more data can be stored in the same volume of diskdrive stack assembly space. It would thus be desirable to make the PZTconstraining layers to be quite thin. However, the thinner the PZTconstraining layers are, the higher the electric field strengths areacross those layers during operation, and hence the more prone they areto being depoled during operation due to too high an electric fieldstrength. Nominally, therefore, the main PZT layer and the constrainingPZT layers should have the same thickness.

One solution to making the constraining PZT layers thinner withoutsubjecting them to depoling is to reduce the strength of the electricfield(s) across the constraining layer(s) using one or more of variouspossible means without significantly reducing the electric field acrossthe main PZT layer. A first means for accomplishing that objective is topattern one or more of the electrodes that is operationally associatedwith one of the active PZT constraining layers but is not operationallyassociated with the main PZT layer, such as by adding holes 3133 inelectrode 3132 or other electrical voids. The patterning could also takethe form of a mesh pattern such as a grid of parallel or intersectingconductors with electrical voids between them. By reducing thepercentage of area of the electrical conductor within the planarelectrode 3132, the electric field strength across constraining layers3130 and 3140 are effectively reduced without reducing the electricfield strength across main PZT layer 3120.

A second solution is to increase the coercive electric field strength ofthe constraining layer(s) so that the constraining layers are moreresistant to depoling. The coercive electric field strength, or simply“coercivity” when referring to a piezoelectric material, is a measure ofhow great an electric field strength is required in order to depole thepiezoelectric material. Making the constraining layer(s) 3130, 3140 havea higher coercivity than the main PZT layer 3120 allows thoseconstraining layers to be made thinner without risk of depoling whensubjected to the same activation voltage as the main PZT layer. Theconstraining layers 3130, 3140 can be made to have higher coercivities,possibly at the price of some loss of d31 stroke length or otherdesirable characteristics, by using a different or slightly differentpiezoelectric material, or by other processing.

Another solution is to reduce the effective voltage that is applied tothe driven electrode associated with the constraining layer(s) by usingsome kind of a voltage reducer such as a voltage divider resistornetwork, a diode, a voltage regulator, or any one of variousfunctionally similar devices which will occur to one of skill in thefield. In the figure, generalized voltage reducer 3144 reduces thevoltage received by electrode 3142, thus reducing the electric fieldstrength experienced by constraining layer 3140 but not by main PZTlayer 3120. The voltage divider can be integrally formed and thusdisposed between the adjacent piezoelectric layers, such as by applyingthe metallization that forms the electrode layer in such as way as toform a voltage divider resistor network on the surface of the PZTmaterial. A simple resistive voltage divider would require a groundwhich could be made available on the same layer. Many constructions arepossible as will be apparent to designers of such devices.

Patterning 3133 and voltage reducer 3144 both decrease the strength ofthe electric field across constraining layer 3140, thus allowingconstraining layer 3140 to be made thinner without unacceptable exposingit to depoling during operation. Either electrode patterning and/or avoltage reducer, and/or some other means for reducing the electric fieldstrength across constraining layers 3130 and/or 3140, could be used. Thepatterning 3133 is integrally formed with electrode 3132 and thus isintegrally formed with, and integrated into the microactuator assembly.A voltage reducer for one of the electrodes could be either integrallyformed with and integrated into the assembly, or could possibly beprovided external to the assembly provided that the associated electrodehas its own electrical lead and is not ganged with the other electrodes.

All three of those solutions discussed above may be applied topiezoelectric microactuators having a single active constraining layer,two active constraining layers such as shown in FIGS. 31-33, or moregenerally n active restraining layers such as illustrated in FIG. 35.

FIG. 34 is a graph showing the stroke sensitivity (in nm/V) ofmicroactuators having one or more active restraining layers according tosimulations for various constraining layer constructions (CLC), with amain PZT layer of 45 μm thick, without any patterning 3133 or voltagereducer 3144 to decrease the electric field strength, for threedifferent constructions:

a) one inactive restraining layer (“passive CLC,” the diamond shapeddata points);

b) one active restraining layer (“single layer,” the square datapoints); and

c) two active restraining layers (“double layer,” the triangular datapoints).

The data indicates that, at least for the parameters that were studied,a PZT microactuator having one active restraining layer acting in theopposite direction as the main PZT layer always produces higher strokesensitivity than one in which the restraining layer is inactivematerial. The highest stroke sensitivity is achieved using multipleactive thin layers of PZT acting as restraining layers (i.e., acting inthe opposite direction as the main PZT layer). Specifically, the higheststroke sensitivity was achieved using two restraining layers that wereeach 5 μm thick, or approximately 11% the thickness of the main PZTlayer. Thus, the constraining layer is preferably less than 50% as thickas the main PZT layer, or more preferably less than 20% as thick as themain PZT layer, or more preferably still within the range of 5-15% asthick as the main PZT layer.

For two active restraining layers, the stroke sensitivity decreasesdramatically with increasing thickness of the restraining layers, withthe highest stroke sensitivity for the case of two active constraininglayers each of about 5 μm thick. Thus, the microactuator preferably hastwo or more restraining layers of a combined thickness that is less thanthe thickness of the main PZT layer, and more preferably their combinedthickness is less than 50% the thickness of the main PZT layer, and morepreferably still each constraining layer is less than half as thick asthe main PZT layer, and more preferably still each constraining layer isless than 20% as thick as the main PZT layer, and more preferably stilleach constraining layer is within the range of 5-15% as thick as themain PZT layer.

For a microactuator assembly having a single active restraining layer,the loss in stroke sensitivity as the restraining layer thicknessincreases was not nearly as dramatic as for the case of two activerestraining layers. A local maxima occurs at about 10 μm thickness forthe single active restraining layer. Thus, for a microactuator assemblyhaving a single active restraining layer, the thickness of that layer ispreferably in the range of 10-40% as thick as the main PZT layer, andmore preferably in the range of about 10-20% as thick as the main PZTlayer.

FIG. 35 is cross sectional view of another embodiment in which themicroactuator assembly comprises multiple active PZT layers, andconceptually showing the poling process and the resulting polingdirections. When the device of FIG. 35 is electrically and mechanicallybonded to a suspension such as shown in FIG. 31 with electrodes 3524 and3528 ganged by conductive epoxy, the result is one main active PZTlayer, and three active PZT layers acting as restraining layers becausethey tend to act in the opposite direction as the main active PZT layer.That is, the bottom PZT layer expands while the top three PZT layerscontract, or vice versa.

The construction of the microactuator assembly can be easily extendedfrom a device having one active main PZT layer and two active PZTrestraining layers as shown in FIGS. 31-33, and one active main PZTlayer and three active PZT restraining layers shown in FIG. 35, to anynumber of active main layers and active restraining layers. The electricfield strength across one or more of the constraining layers can bereduced by various means including electrode patterning and/or a voltagereducer. Experimentation will reveal optimal numbers of constraininglayers and optimal thicknesses for different applications.

The effect of a constraining layer on a PZT to enhance its stroke lengthcan also be achieved by using a multi-layer PZT device in which the topPZT layer acts differently in response to an actuation voltage that doesthe bottom layer(s), and thus acts as a constraining layer, such as inthe embodiments of FIGS. 36-39.

FIG. 36 is a side sectional view of a multi-layer PZT microactuatorassembly 714 according to a further embodiment, in which assembly 714 isa multi-layer PZT device having multiple piezoelectric layers. Thedevice is commonly poled, meaning that a voltage applied to the devicecauses both of the devices to expand, or both to contract. The toppiezoelectric layer 730 is affixed over the bottom piezoelectric layer720, which is the layer closest to the suspension surface to which theassembly is bonded, in stacked fashion. Top piezoelectric layer 730 isthicker than the bottom piezoelectric layer 720. The figure shows thestructure of the device and the voltages applied during poling. Thepoling method uses one positive voltage (Vp+) applied to first electrode726, and a ground (GND) applied to electrode 728 with the polingelectric fields being oriented in opposite directions, and the twoelectrodes being electrically isolated from one another by strips orgaps of non-conductive areas such as gaps in the metallization, on eachof the top and bottom electrodes. This is currently the most commonmethod used for multi-layer PZT polarization. This multi-layer PZT 714is said to be “commonly poled.”

FIG. 37 is a side sectional view of PZT microactuator assembly 714 ofFIG. 36 showing an example of how the assembly might be electrically andmechanically bonded to a suspension. In the figure, the suspensionincluding its electrical circuit is the same as that of FIG. 26,although the bonding for this embodiment uses only conductive epoxy 460and 162, and does not use non-conductive epoxy. In operation, when anactuation voltage is applied across the electrodes 726/728, bothpiezoelectric layers 720, 730 expand in a lengthwise direction, or bothcontract in a lengthwise direction depending on the polarity of theapplied actuation voltage.

Because the top PZT layer 730 is thicker than the bottom PZT layer 720,when the same actuating voltage is applied across both layers the toplayer experiences a smaller electric field across it, and therefore thetop layer does not expand (or contract) as much as does the bottomlayer. The top piezoelectric layer therefore acts as a constraininglayer due to the difference in expansions between the two piezoelectriclayers. The net effect can be that the effective stroke length of thedevice is increased as compared to if the top piezoelectric layer werenot present.

FIG. 41 is a side sectional view of the PZT microactuator of FIG. 36illustrating the assembly installed within a suspension, this time usingthe same bonding technique as shown in FIG. 26.

FIG. 38 is a side sectional view of a multi-layer PZT microactuatorassembly 814 according to a still further embodiment, in which the PZTis a multi-layer PZT and the top PZT layer 830 is an active layer thattends to expand in the opposite direction as the bottom PZT layer 820,which will be referred to as the main PZT layer 820. Assembly 814 hasthree separate electrodes 826/827/828 and corresponding poles duringpoling. The device is said to be reverse poled. The figure shows thestructure of the device and the voltages applied during poling. Thepoling method uses three separate electrodes 826/827/828 and threedifferent voltages: a Vp+ that is applied to the bottom electrode 827, aVp− that is applied to the top electrode 826, and GND that is applied tothe middle or common electrode 828.

In operation, if the same voltage is applied to both the top and bottomelectrodes while GND is applied to the common center electrode, the topPZT layer 830 or restraining layer expands in a lengthwise direction (orcontracts in a lengthwise direction depending on the polarity of theapplied actuation voltage) while the main PZT layer 820 on the bottomcontracts in a lengthwise direction (or expands in a lengthwisedirection), or vice versa. That is, the PZT layers 820 and 830 act ortend to act in opposite directions, with the action of the toppiezoelectric layer at least partially counteracting the action of thebottom piezoelectric layer. This PZT is therefore said to be “reversepoled.”

FIG. 39 is a side sectional view of the PZT microactuator assembly 814of FIG. 38 showing an example of how the assembly might be electricallyand mechanically bonded to a suspension. Conductive adhesive 162 on theright hand side is applied so as to bridge the non-conductive strip orgap 829, thus applying ground to both the top electrode 826 and thebottom electrode 827. In operation, when an actuation voltage is appliedacross the top and bottom electrodes, top PZT layer 830 expands whilethe bottom layer 820 contracts (or vice versa). The top PZT layer thusworks in the opposite direction as the bottom PZT layer. The top PZTlayer thus acts not merely as a passive restraining layer, but as anactive layer actively pushing or pulling in the opposite direction asthe bottom layer.

FIG. 40 is a graph showing the stroke sensitivity of the PZTmicroactuator assemblies of FIGS. 37 and 39 compared to the strokesensitivity of a baseline device as a function of the thickness of thetop PZT layer, with the bottom PZT layer being 45 μm thick for all ofthe traces on the graph. The baseline device (represented by the dashedtrace) is a standard single-layer PZT of 45 μm thickness. That baselinedevice exhibits a stroke sensitivity of 9.0 nm/V. The data are accordingto modeling simulations.

As can be seen from the graphs, for the 3-pole configuration of FIG. 39having reverse poling, the data being represented by the triangularshaped data points, the presence of top PZT layer 830 which is reversepoled dramatically increases the stroke sensitivity as compared to thebaseline device. For example, when the top PZT layer is 15 μm thick, thesuspension stroke sensitivity is 24.5 nm/V, which represents a 172%increase over the baseline of 9.0 nm/V. For this device, the reversepoling configuration having a thin top constraining layer on the orderof 10-15 μm thick, or about 22-33% as thick as the bottom PZT layer,resulted in the highest stroke sensitivity. The stroke sensitivitygenerally decreases with increasing thickness of the top PZT layer. The3-pole configuration of FIG. 39 with a thin reverse poled constraininglayer is therefore generally the most effective and desirableconfiguration. It is anticipated that the top PZT layer is preferably5-50% as thick as the bottom PZT layer, and more preferably about 10-40%as thick as the bottom PZT layer, and more preferably still about 20-35%as thick as the bottom PZT layer.

As can be further seen, a configuration in which the top layer isinactive PZT material, the data being represented by the square shapeddata points, produces the next best stroke sensitivity. The top layercan be made to be inactive by either not poling it and/or by arrangingthe electrodes so that the top layer is not subjected to an electricfield when the overall microactuator device is actuated. Either one ofthose could be accomplished by severing electrode 726 such that thatelectrode does not wrap around to the top of device 714, and hence CLCportion 730 is not activated. The stroke sensitivity of thisconfiguration is higher than for the commonly poled configuration ofFIGS. 36-37 in which an actuation voltage is applied to the top PZTlayer 730 (represented by the circle shaped data points), but is lessthan the reversed poled configuration of FIGS. 38-39. The strokesensitivity starts out higher than for the baseline device, thenincreases with increasing thickness to about 25 μm or about 55% of thethickness of the main PZT layer, then stays fairly constant until about55 μm in thickness or about 122% of the thickness of the main PZT layer,then decreases slightly thereafter with further increases in thethickness of the restraining layer. Thus, an inactive restraining layercan have a thickness of greater than 0, but preferably has a thicknessin the range of 25-55 μm or about 55-125% of the thickness of the activePZT layer.

Finally, the commonly poled configuration of FIG. 37 in which the topPZT layer is thicker than the bottom PZT layer, the data beingrepresented by the tilted square shaped data points, was the worstperforming design overall. For this configuration, the strokesensitivity improved over the stroke sensitivity for the baselinesingle-layer PZT only if the top PZT layer was greater than about 37 μmthick, or about 82% as thick as the main PZT layer. The effective strokelength increased with increasing thickness of the top layer. When thetop PZT thickness was 65 μm, the suspension stroke sensitivity was 29%higher than the baseline of 9.0 nm/V. That 29% increase in strokesensitivity over the baseline represented the best results obtained forthis configuration. For the commonly poled case, therefore the thicknessis preferably at least 80% as thick as the main PZT layer, and morepreferably greater than the thickness of the main PZT layer, and morepreferably still greater than about 125% of the thickness of the mainPZT layer.

A design approach using multi-layer PZT microactuators as shown in FIGS.36-39 which use the top PZT layer(s) employed as a constraining layer(s)can be extended more generally to PZTs having three, four, five, or morelayers, having two, three, four, or more active layers, respectively,acting as constraining layers.

U.S. Pat. No. 9,070,394 to Hahn et al., which is owned by the presentapplicant and which is incorporated herein by reference, discloses inFIGS. 31-35 a multi-layer piezoelectric device in which the constraininglayer comprises an unpoled, inactive layer of piezoelectric material.The inventors have discovered that because poling piezoelectric materialcauses a permanent change to its dimensions, poling less than all layersof a multi-layer piezoelectric device induces unequal dimensionalchanges and hence stresses into the device. It is believed that suchstresses could cause cracking and hence premature failure of the device.

Such stresses in a multi-layer piezoelectric device in which theconstraining layer is a layer of inactive PZT material, can be reducedby poling the constraining layer. In that way, both the active PZT layerand the inactive layer experience similar permanent changes indimensions during poling, thus eliminating the mechanical stressescaused by poling only one layer, or more generally, by poling less thanall layers.

FIG. 42 is a side sectional view of a PZT microactuator 914 according tosuch an embodiment having a poled but inactive constraining layer 930,and showing the poling of the device. In the embodiment PZTmicroactuator 914 includes two PZT layers 920, 930 arranged in a stackedrelation and affixed together such as by being formed integrallytogether as described with respect to prior embodiments. Bottompiezoelectric material layer 920 will become the main or active PZTlayer. Top piezoelectric material layer 930 will become the poled butinactive constraining layer.

A first method of poling the device is to apply a voltage differentialacross the top and bottom electrodes 926, 927 respectively, and to allowmiddle electrode 928 to float. For example, +Vp is applied to topelectrode 926, −Vp is applied to bottom electrode 927, and middleelectrode 928 is allowed to float. That grounding method will result ina uniform electric field being applied across the both piezoelectriclayers 920/930, and hence both piezoelectric layers are poledsimultaneously and to the same extent.

A second method is to apply +Vp and −Vp across those electrodes and toconnect middle electrode 928 to ground as shown in the figure.

More generally, the device may be poled by applying three or moredifferent voltages to the three or more terminals.

In most but not necessarily all cases, equal poling, i.e., poling usingequal electric field strengths and at the same time, will be preferred.In the case of two PZT layers of unequal thickness and three polingvoltage potentials, that would call for applying unequal voltagedifferentials across the different PZT layers, thus producing the sameelectric field strength and hence the same degree of poling across thosedifferent PZT layers. For example, if the two PZT layers are of unequalthickness, polling voltages of +15V, Gnd, and −10V may be the requiredvoltages to produce equal electric field strengths across, and henceequal poling of, those two layers.

FIG. 43 is a side sectional view of the PZT microactuator assembly ofFIG. 42 illustrating the assembly installed within a suspension. Inoperation, an actuation voltage is applied to middle electrode 928; andbottom electrode 927 is grounded through conductive epoxy 162, gold bondpad 469, and stainless steel 154 of the suspension flexure. Topelectrode 926 is not electrically connected to anything and thereforefloats. This produces an actuating electric field across main PZT layer920 causing it to expand or contract, with top PZT layer 930 beinginactive and thus acting as a constraining layer. The presence of thepoled-but-inactive constraining layer 930 acts to increase the effectivestroke length of the device compared to if the constraining layer 930were not present, as described earlier with respect to otherembodiments.

In the exemplary embodiment, the device 914 has a total of threeelectrodes but only the first and second electrodes are accessible fromthe bottom of the assembly. Because the third electrode is a floatingelectrode that is not connected to either a driving voltage or ground,there is no need to make it electrically accessible whether from thebottom of the device or otherwise.

As with other embodiments, the embodiment of FIGS. 42 and 43 is notlimited to two layers. Rather, more generally such a multi-layerpiezoelectric actuator can include multiple poled and active PZT layers,and multiple poled but inactive PZT layers.

Although equal poling will be preferred in most cases, the constraininglayer need not be poled exactly equally with the active piezoelectriclayer. It is believed that it will be generally sufficient for the twolayers to be substantially equally poled. The two layers may be poled towithin 10% of each other, or to within 25% of each other. Similarly, itis not strictly necessary that the constraining layer be completely andtotally inactive. Rather, the constraining layer could have a voltagedifferential applied across it in operation that is less than 10% of theactivating voltage or activating electric field that is applied acrossthe first or active PZT layer, or less than 25% of the activatingvoltage or electric field, or even less than 50% of the activatingvoltage or electric field. Further, although the inventors currentlybelieve that it is generally desirable for the PZT layers to have equalthicknesses, it should be sufficient that the PZT layer havesubstantially equal thicknesses, such as being within 10% of each otheror 25% of each other in thickness.

Although the third electrode 926 can be floating in operation as shownin FIG. 43, an alternative way of ensuring that no electric field orsubstantially no electric field exists across poled but inactiveconstraining layer 930 would be to electrically gang third electrode 926to second electrode 928, thus ensuring that no voltage differentialexists across constraining layer 930.

The PZT microactuators disclosed herein can be used as actuators infields other than disk drive suspensions. Such microactuators and theirconstruction details therefore constitute inventive devices regardlessof what environment they are used it, be that environment the disk drivesuspension environment or any other environment.

It will be understood that the terms “generally,” “approximately,”“about,” “substantially,” and “coplanar” as used within thespecification and the claims herein allow for a certain amount ofvariation from any exact dimensions, measurements, and arrangements, andthat those terms should be understood within the context of thedescription and operation of the invention as disclosed herein.

It will further be understood that terms such as “top,” “bottom,”“above,” and “below” as used within the specification and the claimsherein are terms of convenience that denote the spatial relationships ofparts relative to each other rather than to any specific spatial orgravitational orientation. Thus, the terms are intended to encompass anassembly of component parts regardless of whether the assembly isoriented in the particular orientation shown in the drawings anddescribed in the specification, upside down from that orientation, orany other rotational variation.

All features disclosed in the specification, including the claims,abstract, and drawings, and all the steps in any method or processdisclosed, may be combined in any combination, except combinations whereat least some of such features and/or steps are mutually exclusive. Eachfeature disclosed in the specification, including the claims, abstract,and drawings, can be replaced by alternative features serving the same,equivalent, or similar purpose, unless expressly stated otherwise. Thus,unless expressly stated otherwise, each feature disclosed is one exampleonly of a generic series of equivalent or similar features.

It will be appreciated that the term “present invention” as used hereinshould not be construed to mean that only a single invention having asingle essential element or group of elements is presented. Similarly,it will also be appreciated that the term “present invention”encompasses a number of separate innovations which can each beconsidered separate inventions. Although the present invention has thusbeen described in detail with regard to the preferred embodiments anddrawings thereof, it should be apparent to those skilled in the art thatvarious adaptations and modifications of the present invention may beaccomplished without departing from the spirit and the scope of theinvention. Accordingly, it is to be understood that the detaileddescription and the accompanying drawings as set forth hereinabove arenot intended to limit the breadth of the present invention, which shouldbe inferred only from the following claims and their appropriatelyconstrued legal equivalents.

We claim:
 1. A multi-layer piezoelectric microactuator assemblycomprising: a first piezoelectric layer and a second piezoelectriclayer, each of the first and second piezoelectric layers comprisingpoled piezoelectric material, the first piezoelectric layer being closerto a surface to which the microactuator assembly is bonded than is thesecond piezoelectric layer; a first electrode on a bottom side of thefirst piezoelectric layer; and a second electrode on a top side of thefirst piezoelectric layer and disposed underneath the secondpiezoelectric layer, such that a first voltage differential appliedacross the first and second electrodes induces an electric field acrossthe first piezoelectric layer; wherein: the first electrode isoperatively connected to a first voltage and the second electrode isoperatively connected to a second voltage different from the firstvoltage thereby defining a first voltage differential applied across thefirst piezoelectric layer, the first voltage differential inducing afirst electric field across the first piezoelectric layer causing thefirst piezoelectric layer to expand or contract; and substantially novoltage differential is applied across the second piezoelectric layer;whereby: the second piezoelectric layer is a substantially inactivepiezoelectric layer and acts as a constraining layer to resist expansionor contraction of the first piezoelectric layer; and an overallexpansion of contraction of the microactuator assembly is made to begreater in response to the first voltage differential being appliedacross the first piezoelectric layer than if the second piezoelectriclayer were not present.
 2. The microactuator assembly of claim 1 furthercomprising: a third electrode extending over the second piezoelectriclayer such that the second piezoelectric layer is disposed between thesecond and third electrodes; and the third electrode is a floatingelectrode.
 3. The microactuator assembly of claim 1 wherein: the firstand second piezoelectric layers have equal thicknesses; and the firstand second piezoelectric layers were poled using equal strength electricfields.
 4. The microactuator assembly of claim 1 wherein: the first andsecond piezoelectric layers have thicknesses that differ by less than10%; and the first and second piezoelectric layers were poled usingrespective electric field strengths that differed by less than 10%. 5.The microactuator assembly of claim 1 wherein: the first and secondpiezoelectric layers have thicknesses that differ by less than 25%; andthe first and second piezoelectric layers were poled using respectiveelectric field strengths that differed by less than 25%.
 6. Themicroactuator assembly of claim 1 wherein: the first and secondpiezoelectric layers have different thicknesses; and the first andsecond piezoelectric layers were poled using different voltagedifferentials applied respectively across them.
 7. The microactuatorassembly of claim 1 wherein the second piezoelectric layer has a secondelectric field applied across it that is less than 10% of the firstelectric field applied across the first piezoelectric layer.
 8. Themicroactuator assembly of claim 1 wherein the second piezoelectric layerhas a second electric field applied across it that is less than 25% ofthe first electric field applied across the first piezoelectric layer.9. The microactuator assembly of claim 1 wherein the first and secondpiezoelectric layers were poled such that, for an equal strengthmagnitude electric field applied across both piezoelectric layers, thesecond piezoelectric layer expands or contracts to within 10% of asimultaneous expansion or contraction of the first piezoelectric layer.10. The microactuator assembly of claim 1 wherein the first and secondpiezoelectric layers were poled such that, for an equal magnitudeelectric field applied across both piezoelectric layers, the secondpiezoelectric layer expands or contracts to within 25% of a simultaneousexpansion or contraction of the first piezoelectric layer.
 11. Themicroactuator assembly of claim 1 wherein the first and secondpiezoelectric layers were simultaneously poled using a single electricfield that was applied across both of said first and secondpiezoelectric layers.
 12. The microactuator assembly of claim 1 furthercomprising a third piezoelectric layer disposed between the first andsecond piezoelectric layers, said third piezoelectric layer being apoled and active piezoelectric layer.
 13. A multi-layer piezoelectricmicroactuator assembly comprising a poled and active piezoelectric layerand a poled but inactive piezoelectric layer affixed together in avertically stacked relation.
 14. A multi-layer piezoelectricmicroactuator assembly comprising: a first electrode; a first poledpiezoelectric layer over the first electrode; a second electrode overthe first poled piezoelectric layer; a second poled piezoelectric layerover the second electrode; a third electrode over the second poledpiezoelectric layer; wherein: the first and second electrodes areelectrically accessible from a bottom of the assembly; and the thirdelectrode is not electrically accessible from the bottom of theassembly.
 15. The microactuator assembly of claim 14 wherein: each ofthe first and second electrodes is connected to one of a driving voltageand ground, and the third electrode is not connected to either a drivingvoltage or a ground.
 16. The microactuator assembly of claim 14 whereinthe third electrode is electrically connected to the second electrodewhereby no electric field exists between the second and thirdelectrodes.
 17. A method of using the microactuator assembly of claim 14comprising inducing an electric field across the first piezoelectriclayer whereby the first piezoelectric layer is a poled and activepiezoelectric layer, and not inducing an electric field across thesecond piezoelectric layer whereby the second piezoelectric layer is apoled but inactive piezoelectric layer which tends to resist expansionor contraction of the first piezoelectric layer.
 18. The microactuatorassembly of claim 14 wherein the first and second piezoelectric layershave equal thicknesses and were poled to equal degrees.
 19. Themicroactuator assembly of claim 14 wherein the first and secondpiezoelectric layers have substantially equal thicknesses and were poledto substantially equal degrees.
 20. The microactuator assembly of claim14 wherein the first and second piezoelectric layers were simultaneouslypoled using a single poling voltage differential that was applied acrossthe first and third electrodes.